5.4.2.1.2. Average storage method

To account for dynamic systems-such as afforestation projects, in which planting,
harvesting, and replanting operations take place-an alternative approach has
been used (e.g., Dixon et al., 1991, 1994; Masera, 1995): the average
storage method (Schroeder, 1992). This method entails averaging the amount
of carbon stored in a site over the long term according to the following equation:

where t is time, n is the project time frame (years), and measurements
are expressed in tons of carbon per hectare. The advantage of this method is
that it accounts for the dynamics of carbon storage over the whole project duration,
not only at the times chosen for accounting. This method is also useful for
comparing different projects with different growth patterns. As Figure
5-3 shows, the average storage over three rotations of Project 1 is higher
than that of Project 2. A weakness of this method, however, relates to the still
subjective time frame, n, chosen for running the analysis. In the case of Figure
5-3, for example, the average net carbon storage in either project would
be equal whether the calculation was performed for one, two, or an infinite
number of rotations, as long as the denominator chosen for the foregoing equation
coincided with the last year of a rotation.

Figure 5-3: Projection of carbon stored in
two tree plantation projects over three rotations. For simplicity, it
is assumed that the baseline is zero, that harvesting leads to an immediate
release of all carbon stored, and that equilibrium of carbon pools is
reached in the first rotation cycle. The curves illustrate carbon storage
over time; the straight horizontal lines show the average storage calculated
for the two projects.